This invention relates to a planetary-roller transmission which transmits power by making use of friction developed by rollers in rolling contact.
A conventional transmission system of this character, in an application as a speed reducer, is shown in FIGS. 1 and 2. The numeral 1 designates a sun roller directly mounted on an input shaft 8 which is driven for rotation. 3 is an internal contact ring fixedly secured to a casing 10, and 2 is a plurality of (in this case, three) planet rollers therein rotatably supported, through bearings 4, by planet pins 5, all of which in turn are held by a common carrier 6. 11 and 12 are bearings.
The sun roller 1, the plurality of planet rollers 2, and internal contact ring 3 coact to transmit power through the agency of a frictional force U that results from radial bearing, with a contact pressure P, of the rollers against the encircling ring.
In this arrangement, as indicated in FIG. 2, the components assembled are so made that the inside diameter D.sub.i of the internal contact ring 3 is slightly smaller than the sum of the outside diameter D.sub.s of the sun roller 1 and twice the outside diameter D.sub.p of the planet rollers 2 (that is, D.sub.s +2D.sub.p). It is by the forced contact between the ring and rollers with elastic deformation that the contact pressure P is produced. The arrangement presents the following difficulties:
(1) With the planetary-roller transmission of the type described, the contact pressure P is usually set to a constant predetermined value and hence the frictional force U through the agency of which the power is transmitted is constant, too. Therefore, the transmission is devoid of clutch functions for drivingly connecting or disconnecting two working parts. If the transmission is subjected to an overload, the planet rollers 2, sun roller 1, and internal contact ring 3 can be seized due to excessive slippage between their contacting surfaces, making the operation no longer possible.
(2) Generally, the transmission of this type is seldom used alone; in most cases the mechanism is incorporated as an element in one of varied power-transmitting circuits. In the latter cases, from the viewpoints of machine performance, safety, and ease of maintenance and inspection, if is often necessary for the practical purposes to cut off or reconnect the transmission of power temporarily at the front or rear of the mechanism while in or out of operation. In such an event, the requirement will not be met because the conventional transmission lacks the power cutoff or reconnecting, or clutching, functions.
In order to overcome these problems, modified arrangements as illustrated in FIGS. 3 and 4 have been provided.
The system shown in FIG. 3 has a clutch 73 interposed between input shafts 8a and 8b, so that the power from the input shaft 8a is delivered to the transmission or cut off by the engaging or disengaging action of the clutch 73.
In that event, the installation of the clutch 73 calls for an added mounting space Lh axially of the transmission. Moreover, the manufacturing cost of the clutch 73 naturally raises the total cost of the transmission.
In FIG. 4 the clutch 74 is located between the internal contact ring 3 and the casing 10 and the engaging or disengaging action of the clutch locks the internal contact ring 3 or sets it free.
In this case the radial dimension Lv of the transmission is increased, again adding appreciably to the overall cost of the mechanism.
Parts in FIGS. 3 and 4 like or similar to those in FIGS. 1 and 2 are indicated by like numerals.